1. Field of the Present Invention
The present invention relates to a method for making a hindered phenolic antioxidant, and more particularly, to a method for making a hindered phenolic antioxidant based on Octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate.
2. Description of Related Art
Octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate is a hindered phenolic antioxidant extensively applied to industries of plastic, synthetic fibers, rubber and petrochemistry as an excellent antioxidative additive.
A conventional method for making Octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate involves, in the presence of a catalyst, reacting methyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate with 1-Octadecanol in a transesterification process and, during the transesterification process, continuously removing methanol by-products from the reactants by vaporization or other means, thereby improving the conversion rate in the transesterification process. The crude product Octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate produced as a result of the transesterification process is then crystallized, filtered and dried to obtain the product of Octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate. To make Octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, the quality and manufacturing cost are subject to the conversion rate in the transesterification process and the color of the product. Thus, it is well recognized that the conversion rate and the color of the crude product obtained through the transesterification process are quite important.
In early days, acid catalysts (such as sulfuric acid and p-toluene sulfo acid) or alkaline catalysts (such as sodium hydroxide and triethylamine) might be used in the transesterification process for making Octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate. Nevertheless, acid catalysts or alkaline catalysts used in the transesterification process is disadvantageous because of poor performance in reaction, corrosiveness to equipments, and complicated neutralization and washing procedures in order to remove the catalysts after transesterification reaction, thus being inconvenient and difficult to use. To overcome these defects, a transesterification catalyst is made from a neutral organic heavy metal that has good performance in reaction and non-corrosive (such as (C4H9)2Sn(C2H3O2)2). Yet, heavy metal catalysts thus made bring a new problem—the residual heavy metal of Sn may still be left in the product after purification procedures, including crystallization, filtering and drying, have been carried out following the transesterification process.
In recent years, due to people's increasing concern about environmental protection and health care, it is a common request that the finished products of Octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate shall not contain heavy metal residuals. Hence, it is commonly believed that non-heavy metal is the safer catalyst for transesterification reaction, and that the amount of the catalyst used shall be small enough to spare the complicated procedures for catalyst crystallization, washing and extraction. Alkali metal methoxide (such as lithium methoxide (CH3OLi), sodium methoxide (CH3ONa) and potassium methoxide (CH3OK)) is thus identified as the non-heavy metal catalyst for transesterification reaction because it contains no heavy metal and is efficient in transesterification process even when a small amount of it is used.
Alkali metal methoxide exists in a solid state with a high melting point and is unlikely to be liquefiable. Typically, alkali metal methoxide is dissolved in methanol to form a methanol solution containing alkali metal methoxide and then the methanol solution can be used in transesterification reaction.
Prior arts using alkali metal methoxide as catalysts for transesterification reaction have been taught by U.S. Pat. Nos. 3,784,578 and 5,710,316 for making diallyl phthalate. Therein, U.S. Pat. No. 5,710,316 disclosed that drying the reactant (i.e. an alkyl ester/allylic alcohol mixture) by distillation to less than about 200 ppm water, the transesterification reaction can be achieved by a heating process with the solution of sodium methoxide in methanol/allyl alcohol used in a small amount (less than about 500 ppm based on the amount of the reactant). The methanol by-products are removed overhead by distillation and at last the diallyl phthalate related products, preferably having both high conversion rate and low color, can be obtained.
Similarly, when the method of U.S. Pat. No. 5,710,316 is applied to a process where short linear alcohols are taken as reactants, such as a case where methyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate and pentaerythritol whose linear chain contains alkyl of 3 carbon atoms are transesterification reactants for making pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate), the crude product obtained after the transesterification process is advantageous by its high conversion rate and low color, as affirmed in the patent. After the crude product receives a purification process including crystallization, filtering and drying, the product having high purity and low color can be obtained.
However, when the method of U.S. Pat. No. 5,710,316 is applied to a process where long linear alcohols are taken as reactants, such as a case where methyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate and 1-Octadecanol whose linear chain contains alkyl of 18 carbon atoms are transesterification reactants for making Octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, the crude product of Octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate after the transesterification process is nevertheless disadvantageous for its low conversion rate and high color. Even after crystallization, filtering and drying, the crude product cannot be turned into a product having high purity and low color. Therefore, the method is unfavorable to mass manufacture.
Through researches and repeated experiments, the inventor of the present invention draws a conclusion that by filtering a methanol solution that contains alkali metal methoxide with a filter device with a filter pore diameter of less than 50 μm so as to remove insoluble matters from the methanol solution before using it as a catalyst in transesterification reaction between methyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate and 1-Octadecanol, a crude product of Octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate having high conversion and low color can be obtained. Then by treating the crude product with a purification process for crystallization, filtering and drying, a final product of Octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate having high purity and low color can be obtained. Hence, given the aforesaid method, Octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate can be produced by mass production.